In the fabrication of semiconductors, processes based on the bonding of a thin film followed by the transfer of said film onto a substrate are frequently encountered.
In particular, the SmartCut™ process comprises, in a first step, the formation in what is called a donor substrate of a weakened zone defining the boundary of a thin film intended to be transferred onto what is called a receiver substrate.
Then, after the donor substrate has been bonded to the receiver substrate, the donor substrate is split so as to transfer the thin film onto the receiver substrate.
As a general rule, the splitting is initiated at a predetermined point on the periphery of the donor substrate, followed by propagation in the form of a splitting wave into the rest of the substrate. The splitting is initiated for example by means of mechanical forces, by localized heating, etc.
When splitting substrates, which are in the form of circular wafers, it is frequently found that there is a defect, called a “process line” or a “stress line” which is initiated around a notch located on the edge of the wafer.
This notch, which is generally triangular and penetrates about 2 mm (along the diameter of the wafer), enables the orientation of the wafer to be identified during the various steps of the process. The notch, which is made by the wafer manufacturer before the wafer is cut from the ingot, enables the crystal orientation of the substrate to be identified.
A photograph of such a line associated with the notch is shown in FIG. 1. The notch is identified by the reference sign 4 and the process line is surrounded by dots.
From analyses carried out by the Applicant, this line corresponds to the tearing of the thin transverse film, initiated around the notch by localized stresses.
This is an elongate defect characterized by a high roughness over its entire length, generally a through-defect in its most pronounced central part (the transferred film being torn over its entire thickness), so that a chip constructed on top of this line would not operate.
This line is therefore an unacceptable defect leading to the wafer being scrapped if it is observed to have a length greater than a few microns.
A first object of the invention is therefore to improve the process for fabricating a film transfer structure so as to prevent the formation of such a line around the notch on the final structure.
Another object of the invention is to define a method of splitting a substrate that reduces the appearance of a line associated with the notch.